I've been using clip-ons for a few years and they seem to be Ok. Oh, and the only type of sunnies I've ever used since as long as I can remember even before I started flying nearly thirty years ago have been polarised - both the normal type of sunnies before I needed glasses to now with the clip-ons.
I have three pairs of glasses. One distance/general use pair which I have never used since getting the multifocals, one reading pair which I use for serious reading and computer work and one multifocal pair or probably what has been referred to earlier as varifocal, I use all the time and they are photochromatic but the amount they darken even out in full sun is not enough, and they are too slow to change as well - both to and from clear to tint and back. Even in the cockpit of the aircrafts I fly with their perspex windows they don't darken so I always use the clip-ons, and I have very little problems with the polarisation.
Yes I do sometimes need to tilt my head slightly to see a display better but at no time under normal ops do I ever have a display "go black" because of the polarisation.
And as stated the clip-ons do and have scratched the surface as careful as I have been in putting them on and taking them off. Might be time to get my eyes checked again and get new unscratched lenses.